This invention relates to lithographic printing plates. More particularly, it relates to lithographic plates having on a roughened substrate a substantially conformally coated photosensitive layer and a substantially conformally coated overcoat.
Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties. The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate. Conventionally, the actinic radiation is from a lamp (usually an ultraviolet lamp) and the image pattern is generally determined by a photomask that is placed between the light source and the plate. With the advance of laser and computer technologies, laser sources have been increasingly used to directly expose a printing plate according to digital imaging information.
Currently, most commercial lithographic plates require a development process after the plates being exposed and before put on press. A liquid developer is used to dissolve and clean off the non-exposed areas (for negative plate) or the exposed areas (for positive plate). Such a development process is time and labor consuming and generates wet waste. It would be desirable that such a tedious development process can be eliminated.
On-press developable lithographic printing plates have been disclosed in the literature. Such plates can be directly mounted on press after exposure to develop with ink and/or fountain solution during the initial prints and then to print out regular printed sheets. No separate development process before mounting on press is needed. Among the on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5,407,764, 5,516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, and 6,014,929.
Lithographic plates having a photo-insensitive overcoat on the photosensitive layer have been disclosed in the patent literature, including U.S. Pat. Nos. 5,677,110, 5,599,650, 5,677,108, and 5,997,993. The overcoat is introduced to, for example, reduce physical damage or contamination to the photosensitive layer during handling, improve photospeed, curing efficiency, and curing consistence by reducing oxygen inhibition (for free radical polymerizable photosensitive layer), improve humidity stability, and/or improve on-press developability (for on-press developable plate).
The photosensitive layer is generally coated onto a smooth or grained substrate at sufficient thickness to form a flat, smooth surface. For plates with an overcoat, the overcoat is generally coated to form a flat, smooth surface. While plates with a flat, smooth surface are very useful, they often suffer from the problem that the photosensitive layer surface tends to block to the back of another plate at extreme environmental condition, such as high temperature, high pressure, and high humidity. Also, such a plate can suffer from tackiness or fingerprinting when the photosensitive layer or the overcoat is a semisolid material, is formulated with higher content of liquid components, or is moisture sensitive (at higher humidity). Therefore, there is a desire for a lithographic plate that has excellent block resistance, non-tackiness, and fingerprinting resistance, in addition to on-press developability (for on-press developable plate) and press durability.
I have found that a lithographic plate comprising on a roughened substrate a substantially conformally coated photosensitive layer and a substantially conformally coated overcoat can provide no or low tackiness and excellent block resistance, while allowing excellent press durability. For on-press developable lithographic plate, such a photosensitive layer configuration also allows excellent on-press developability. The photosensitive layer is substantially conformally coated on the roughened substrate surface and the overcoat is substantially conformally coated on the photosensitive layer in a way so that the surface of the overcoat has peaks and valleys substantially corresponding to the major peaks and valleys of the substrate microscopic surface. It is very surprising that such a plate surface gives very low tackiness and excellent block resistance even with a semisolid photosensitive layer and/or semisolid overcoat which is very tacky and has poor block resistance when coated to form a smooth surface.
It is an object of the present invention to provide a lithographic printing plate with no or low tackiness and good block resistance while maintaining excellent press performance.
It is another object of the present invention to provide a lithographic plate having a semisolid photosensitive layer and/or overcoat with no or low tackiness and good block resistance.
It is another object of the present invention to provide a lithographic plate comprising on a roughened substrate a substantially conformally coated photosensitive layer and a substantially conformally coated overcoat in order to achieve reduced tackiness or good block resistance, while allowing excellent press durability.
It is another object of the present invention to provide an on-press developable lithographic plate having on a roughened substrate a conformal photosensitive layer and a conformal overcoat.
Further objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments.
According to the present invention, there has been provided a lithographic printing plate, comprising in order:
(a) a substrate with roughened surface comprising, at least, peaks and valleys;
(b) a photosensitive layer capable of hardening or solubilization upon exposure to an actinic radiation and exhibiting an affinity or aversion substantially opposite to the affinity or aversion of said substrate to at least one printing liquid selected from the group consisting of ink and an abhesive fluid for ink; and
(c) an overcoat;
(d) wherein said photosensitive layer is substantially conformally coated on the substrate surface and said overcoat is substantially conformally coated on the photosensitive layer surface, so that the surface of said overcoat has peaks and valleys substantially corresponding to the major peaks and valleys of the substrate microscopic surface.
According to another aspect of the present invention, there has been provided a method of lithographically printing images on a receiving medium, comprising in order:
(a) providing a lithographic printing plate comprising: (i) a substrate with roughened surface comprising, at least, peaks and valleys; (ii) a photosensitive layer capable of hardening or solubilization upon exposure to an actinic radiation and exhibiting an affinity or aversion substantially opposite to the affinity or aversion of said substrate to at least one printing liquid selected from the group consisting of ink and an abhesive fluid for ink, the non-hardened or solubilized areas of said photosensitive layer being soluble or dispersible in ink (for waterless plate) or in ink and/or fountain solution (for wet plate); and (iii) an ink and/or fountain solution soluble or dispersible overcoat; wherein said photosensitive layer is substantially conformally coated on the substrate surface and said overcoat is substantially conformally coated on the photosensitive layer surface, so that the surface of said overcoat has peaks and valleys substantially corresponding to the major peaks and valleys of the substrate microscopic surface;
(b) imagewise exposing the plate with an actinic radiation to cause hardening or solubilization of the exposed areas; and
(c) contacting said exposed plate with ink (for waterless plate), or with ink and/or fountain solution (for wet plate) on a printing press to remove the overcoat and the non-hardened or solubilized areas of the photosensitive layer, and to lithographically print images from said plate to the receiving medium.
The plate can be sensitive to an ultraviolet, visible, or infrared radiation. It can be exposed with an actinic radiation through a photomask or by direct exposure according to digital imaging information. For direct exposure, a laser (including infrared, visible or ultraviolet laser) is a preferred radiation source.
For on-press developable plate, the plate can be imaged off press and then mounted on press for on-press development with ink and/or fountain solution and lithographic printing. Alternatively, the plate can be imagewise exposed with a laser while mounted on a plate cylinder of a lithographic press, on-press developed on the same plate cylinder with ink and/or fountain solution, and then directly print images to the receiving sheets.